The percent of high severity defects is another good metric to be tracked. It take ratio between issues with high severity (e.g. Blocker, Critical, Major) and all bugs that are raised for the evaluated period.
% of High Severity Defects = ( # Defect – Blocker, Critical, Major / # All Defects ) * 100
Check following steps by step KPI configuration utilizing the Performance Objectives JIRA dashboard gadget , JQL Data segmentation and custom formula metric.